For many, a visit to the dentist is an anxiety-inducing affair, and the noise of the dental drill is a significant cause of this anxiety. Accordingly, a device which can reduce the perceived sound volume of the drill is also likely to be useful in reducing levels of anxiety. Ideally, such a device will also allow the patient to be able to listen to their own music (for example played on an MP3 player) and/or continue to communicate with the dentist. Conventional noise-cancelling headphones are designed to deal with noise frequencies well below 1 kHz, such as that experienced inside aircraft cabins. Dental drills rotate at speeds reaching beyond 200,000 rpm, and produce noise peak frequencies in the region of 3-6 kHz that are of relatively high power and hence very distinctive. Passive ear defenders work well in reducing sound amplitude at such high frequencies, but they do not effectively address the issue of simultaneously maintaining communication between the dentist and the patient. The simultaneous provision of noise suppression and communication requires a more sophisticated system. EP 0933007 A1 (Syracuse University) discloses an ear muffler device comprising a pair of muffler tubes in the form of headphones. The muffler tubes are configured to isolate the ear canal from ambient air and to reduce the acoustic impedance at the entrance of the ear canal across a wide range of frequencies. However, the device is bulky and looks uncomfortable and unattractive to wear.
US 2006/0042867 A1 (Haussmann et al.) discloses a hearing protection ear plug to be worn in the ear canal having an acoustic filter element and a resonance cavity which is individually designed for the acoustic requirements of the user. Accordingly, such a device would be impracticable as a mass-manufactured item for use by a plurality of users. U.S. Pat. No. 4,437,538 A1 (Ohlsson et al.) discloses an ear cap which includes a cushion formed of an elastic and porous material in combination with a perforated disk defining a cavity to attenuate the sound passing through the device.
A number of active (electronic) noise cancelling devices are disclosed in WO 02/100287 A2 (Zilberman et al.) and US 2009/0010447 A1 (Waite et al.).
There is a need for a noise cancelling device which is smaller in size than prior art devices, passive (and therefore easy and inexpensive to operate) and which can optionally allow the user to continue to listen to a personal music player and/to the dentist. It will be appreciated that such a device would have noise cancelling uses which go beyond those of a dental patient.